Consider removing syn and proc-macro2

[Yevhenii Reizner]

I'm not familiar with procedural macros, but is it possible to depend only on proc-macro? The problem is that proc-macro2 and syn a very slow to build:

> cargo bloat --release --time -j1
Compiling ...

  Time Crate
44.36s syn
21.22s gumdrop_derive
 5.42s proc_macro2
 1.55s quote
 0.48s gumdrop
 0.12s unicode_xid

On the other hand, arguments parsing without macros is too verbose.

[Murarth]

syn cannot be removed, at least not without replacing it with something that performs its essential functions.

syn parses the Rust syntax of any struct/enum declarations that are decorated with the derive(Options) attribute and parses the content of any #[options(default_expr = "...")] attribute as a Rust expression. This requires compiling syn with the full feature, to enable parsing all valid Rust syntax.

[Yevhenii Reizner]

I see. Thanks.